Project Abstract The long-term goals of the proposed project are to understand the complicated origins of the spinal cord injury (SCI) induced muscle paresis, atrophy, weakness, and muscle contractures that often result in a limited range of joint motion and progressive changes in intrinsic muscle mechanical properties. In particular, our proposed research plan relies on several recently developed novel surface electromygram (EMG) techniques to investigate controversial concepts about caudal motoneuron degeneration after a spinal injury. Using motor unit number index measurements and high-density surface EMG recording and processing techniques, we will determine whether motor unit structure and function changes of paralyzed muscles, potentially due to caudal motoneuron degeneration, can be captured via three specific aims. Namely, we plan to assess whether there is evidence for motoneuron degeneration caudal to spinal injury using estimates of motor unit numbers in paralyzed muscles (Aim 1), and using the recorded spontaneous EMG activities in resting paralyzed muscles (Aim 2). In addition, we will examine alterations in motor unit action potential propagation patterns and innervation zones, as a marker of motoneuron loss and subsequent muscle fiber reinnervation in paralyzed muscles, thus providing secondary evidence for motoneuron degeneration caudal to spinal injury (Aim 3). The methods used in this study are noninvasive, potentially convenient to use, and offer valuable information beyond that provided by needle-based EMG and other routine electrophysiological methods. The findings from these novel methods will enhance our understanding of the pathophysiology of potential motoneuron degeneration in SCI. This will provide guidance for the development of rehabilitation strategies and devices for restoration of normal muscle functions. The findings from this study also have important clinical value for the diagnosis and treatment of spinal injury, improve outcome measurements, and help evaluate the effects of medication or therapies.